Involvement of protein kinase C and intracellular Ca2+ in goldfish brain somatostatin-28 inhibitory action on growth hormone release in goldfish

Goldfish brain somatostatin-28 (gbSS-28) is present in brain and pituitary tissues of goldfish. We assessed whether gbSS-28 targets Ca2+ and/or protein kinase C (PKC)-dependent signaling cascades in inhibiting growth hormone (GH) release. gbSS-28 decreased basal GH release from primary cultures of dispersed goldfish pituitary cells and intracellular free calcium levels ([Ca2+]i) in goldfish somatotropes. gbSS-28 partially reduced [Ca2+]i and GH responses induced by two endogeneous gonadotropin-releasing hormones (GnRHs), salmon (s)GnRH and chicken (c)GnRH-II. Furthermore, gbSS-28 reduced GH increases and abolished [Ca2+]i elevations elicited by two PKC activators, tetradecanoyl 4β-phorbol-13-acetate and dioctanyl glycerol. The PKC inhibitors Gö6976 and Bis II abolished [Ca2+]i responses to PKC activators, but only attenuated GnRH-induced increases in [Ca2+]i and did not alter basal [Ca2+]i. In cells pretreated with Bis II, gbSS-28 further reduced basal [Ca2+]i. Our results suggest that gbSS-28 inhibits GnRH-induced GH release in part by attenuating PKC-mediated GnRH [Ca2+]i signals. gbSS-28 reduces basal GH release also via reduction in [Ca2+]i but PKC is not involved in this regard.